1. Lewis Structures of Covalent Compounds
Learning Target:
I can represent the structure of a molecule by drawing bonds and unshared pairs.
Criteria for Success:
I can construct electron dot structures to illustrate covalent bonds.

2. Lewis Structures of Covalent Compounds
A. Lewis Structures are formulas used to model what atoms look like in a compound that contains atoms that are covalently bonded together.

3. Lewis Structures of Covalent Compounds
Remember that in a covalent bond, nonmetal atoms share electrons in order to obtain a full valence shell.

4. Lewis Structures of Covalent Compounds
The idea that nonmetals will share electrons to get eight valence electrons and be stable is part of the octet rule.

5. What’s in a Lewis Structure
1. Element symbols represent the atom’s core Valence electrons are then represented using either dot-pairs or dashes.

6. What’s in a Lewis Structure
a. If the valence electrons are not involved in bonding they are represented using a dot. These represent unshared pairs of electrons.

7. What’s in a Lewis Structure
b. If the valence electrons are involved in bonding they are represented using a dash between two atomic symbols. These represent shared electron pairs in covalent bonds.

8. What’s in a Lewis Structure
Remember that covalent bonds are formed when nonmetals share valence electrons in order to mutually achieve full valence shells (to be stable like noble gases).
Only valence electrons that are already unpaired will be shared from one atom to another.

9. Unpaired Electron Sharing Example 1: CH4

10. Unpaired Electron Sharing Example 2: NH3

11. Unpaired Electron Sharing Example 3: OF2

12. Unpaired Electron Sharing Example 4: FOCN

13. How do I Lewis
Notice that a pattern shows up for covalent bonds with certain atoms. Certain atoms are most stable with a specific number of bonds and a specific number of unshared (lone) pairs of electrons.

14. Element # bonds # lone pairs H C N O All Halogens All Noble Gases
Patterns in bonding--Atoms in the same groups as C, N, and O will tend to follow the same pattern, but they may not always.
Element 
# bonds 
# lone pairs  H  1  0  C  4  N  3  O  2 
All Halogens 
All Noble Gases  4

15. B. Steps to writing a Lewis structure of a neutral covalent compound
Choose a central atom.
a. If carbon is in the molecule, it will be the central atom.
b. Otherwise, the atom that is capable of having the most bonds will most likely be the central atom.

16. B. Steps to writing a Lewis structure of a neutral covalent compound
c. It is possible for all atoms in a molecule to be bonded in a straight line, like Example 4 earlier. In this case, there is not one sole central atom. The formula for the molecule will generally be written by listing the elements in the order they are chained in the atom. Ex: HSCN, CH3COOH

17. Draw dat Lewis structure
2. Write the symbol for the central atom. Then write the symbols for the other atoms around the central atom Draw the Lewis structures for the individual atoms. 4. Figure out how to “connect the dots.”

18. Draaaaaaawwwwwiiiiiiinnnnng
5. Draw dashes between atoms to show bonding pairs of electrons. Be sure to show the lone pairs around atoms that have them. 6. Double check that each atom has a full octet by counting both bonds and lone pairs as two electrons each belonging to the atom.

19. C. Multiple bonds, expanded octets, and other exceptions
1. Only certain atoms can form double and triple bonds. Generally, they are the “PCONS,” although others can exist.

20. C. Multiple bonds, expanded octets, and other exceptions
2. Some atoms in the third and higher energy levels can hold electrons in their otherwise empty d orbitals. This allows them to form more than 4 bonds and have more than a full octet.

21. C. Multiple bonds, expanded octets, and other exceptions
2. This is most common with P and S, although any nonmetal that has access to d orbitals (so, nonmetals in the third period and below) are capable of doing it. These can have 8, 10, or 12 valence electrons in a molecule.

22. C. Multiple bonds, expanded octets, and other exceptions
2. If you are ever drawing a molecule that has lone pairs and unsure of where else to put them, the lone pairs should go on the expanded octet atom.

23. C. Multiple bonds, expanded octets, and other exceptions
3. Boron is stable with only six electrons, rather than a full octet of eight. This means it can be stable with three bonds and zero lone pairs.

24. C. Multiple bonds, expanded octets, and other exceptions
4. Some noble gases, under extreme laboratory conditions, can be forced to form bonds. This has only been shown to occur with the largest nonradioactive noble gases (Xe and Kr) and the most electronegative atom (F).

25. C. Multiple bonds, expanded octets, and other exceptions
4. Fluorine is able to so strongly attract electrons to itself that it can attract valence electrons in noble gases that have a lot of shielding.

26. C. Multiple bonds, expanded octets, and other exceptions
5. Lewis structures for polyatomic ions can be drawn using generally the same process as covalent molecules.

27. C. Multiple bonds, expanded octets, and other exceptions
5. However, the final structure must have square brackets drawn around it and the charge must be written outside the brackets as a superscript in the upper right corner.

28. D. How to tell if you’re dealing with an exception—NAS charts
1. An NAS chart can be made for any covalent molecule. It has a built-in way to tell if you need to draw a double or triple bond or if you are dealing with an expanded octet.

29. How to tell if you’re dealing with an exception—NAS charts
2. An NAS chart keeps track of the total number of electrons that are Needed for each to be stable with a full “octet” (2 for H, 6 for B, 8 for everything else).

30. How to tell if you’re dealing with an exception—NAS charts
3. An NAS chart also keeps track of the total number of valence electrons Available. If you are trying to draw a polyatomic ion, the charge must be added to this line.

31. How to tell if you’re dealing with an exception—NAS charts
3. (Remember a negative charge means added electrons, so you add that number to the A line. A positive charge means electrons were removed, so subtract that number from the A line.)

32. How to tell if you’re dealing with an exception—NAS charts
4. Finally, an NAS chart calculates how many Shared electrons will have to be in the molecule—this number divided by 2 tells you how many bonds to draw.

33. Example 1: CH4 again C H
Needed
Available
Shared

34. Needed: 2 for H, 6 for B, 8 for everything elseC H
Needed
Available
Shared

35. Needed: 2 for H, 6 for B, 8 for everything elseC H
Needed 8 2
Available
Shared

36. Needed: 2 for H, 6 for B, 8 for everything elseC H
Needed 8 2 16
Available
Shared

37. Available: Number of valence electronsH
Needed 8 2 16
Available
Shared

38. Available: Number of valence electronsH
Needed 8 2 16
Available 4 1
Shared

39. Available: Number of valence electronsH
Needed 8 2 16
Available 4 1
Shared

40. Shared: Needed minus Available = # electrons being shared within bonds8 2 16
Available 4 1
Shared
N - A

41. Shared: Needed minus Available = # electrons being shared within bonds8 2 16
Available 4 1
Shared
N - A

42. Shared: Divide by 2 to get the number of bondsC H
Needed 8 2 16
Available 4 1
Shared
N - A
8 shared
= 4 bonds

43. Let’s draw it!
The NAS chart tells you that the molecule needs 4 total bonds. If you draw the central atom (C) below with the other atoms (4 H’s) around it, you know that each H must have at least one bond to C.

44. Still drawing…
That takes care of all four bonds. You also need to check that all of the available electrons (8) were used. Count each bond as 2 electrons and each lone pair as 2 electrons.

45. Check yoself
The 4 bonds x 2 electrons each = 8 electrons used in your CH4 structure. All 8 available electrons were used, so you know the structure is probably correct.

46. Before you wreck yoself
Check it against the element/#bonds/# lone pairs chart to confirm.
Element 
# bonds 
# lone pairs  H  1  0  C  4  N  3  O  2 
All Halogens 
All Noble Gases  4

47. Don’t worry, it gets more complicated
Don’t worry, it gets more complicated. You won’t always be this bored with how easy it is.
Example 2: OF2 again Make an NAS chart. Need: 8 electrons each because neither O nor F are exceptions.

48. Needed: 2 for H, 6 for B, 8 for everything else
Available
Shared

49. Needed: 2 for H, 6 for B, 8 for everything else24
Available
Shared

50. Available: # valence electronsF
Needed 8 24
Available
Shared

51. Available: # valence electronsF
Needed 8 24
Available 6 7
Shared

52. Available: # valence electronsF
Needed 8 24
Available 6 7 20
Shared

53. Shared: Needed - AvailableO F
Needed 8 24
Available 6 7 20
Shared
N – A

54. Shared: Needed - AvailableO F
Needed 8 24
Available 6 7 20
Shared
N – A 4

55. Shared: Needed - AvailableO F
Needed 8 24
Available 6 7 20
Shared
N – A
4 shared 2
= 2 bonds

56. Draw draw draw your structure gently on your page
Knowing that O is the central atom because it is capable of forming the most bonds, you can draw O with one F on either side. Each F must be bonded to the O at least once. Drawing those bonds will show both of the bonds that the NAS chart requires.

57. We’re not done, we’re not done, we’re not done yet. Don’t get in a rage!
However, that only takes care of 4 electrons (2 per bond x 2 bonds = 4) and there are 20 available. All the other available electrons must be written in as lone pairs. Follow the element/# bonds/# lone pairs chart to draw them out.
Element
# bonds 
# lone pairs  H  1  0  C  4  N  3  O  2 
All Halogens 
All Noble Gases  4

58. Check/don’t wreck
Double check that each element gets a full octet to confirm you’ve drawn it correctly.

59. Let’s try something totally new
Example 3: CO2 Make an NAS chart. Need: 8 electrons each because neither C nor O are exceptions.

60. Needed: 2 for H, 6 for B, 8 for everything elseC O
Needed 8
Available
Shared

61. Needed: 2 for H, 6 for B, 8 for everything elseC O
Needed 8 24
Available
Shared

62. Available: # valence electrons
Needed 8 24
Available 4 6
Shared

63. Available: # valence electrons
Needed 8 24
Available 4 6 16
Shared

64. Shared: Needed - AvailableC O
Needed 8 24
Available 4 6 16
Shared
N – A

65. Shared: Needed - AvailableC O
Needed 8 24
Available 4 6 16
Shared
N – A

66. Shared: Needed - AvailableC O
Needed 8 24
Available 4 6 16
Shared
N – A
8 shared 2
= 4 bonds

67. How do I draaaawww without you
Draw C, the central atom, with an O on either side. Each O must be bonded to C at least once, so you can draw in those bonds.

68. Don’t you wish your girlfriend could draw like me
That only takes care of two bonds, and the NAS chart says you need 4. C and O are each part of the PCONS, so you know they are capable of making multiple (double or triple) bonds.

69. My drawing brings all the boys to the yard
Add bonds between each O and C to create double bonds on each side of C. This satisfies the 4 bonds needed.

70. A whole neeeewww structuuuuure
However, all 16 available electrons need to be visible on the structure, and currently only 8 are.

71. A new fantastic molecuuuule
So lone pairs need to be added to make sure that each atom has a full octet, and to satisfy the element/#bonds/# lone pairs chart. Since only 8 of the available 16 electrons are in bonds, the other 8 available electrons must all be in lone pairs on the various atoms.
Element 
# bonds 
# lone pairs  H  1  0  C  4  N  3  O  2 
All Halogens 
All Noble Gases  4

72. No one to tell us no, go check yourself

73. Or say you’re only basic
Example 4: SF6 Make an NAS chart. Need: 8 electrons each because neither S nor F are exceptions.

74. Needed: 2 for H, 6 for B, 8 for everything else
Available
Shared

75. Needed: 2 for H, 6 for B, 8 for everything else56
Available
Shared

76. Available: # valence electronsF
Needed 8 56
Available 6 7
Shared

77. Available: # valence electronsF
Needed 8 56
Available 6 7 48
Shared

78. Shared: Needed - AvailableF
Needed 8 56
Available 6 7 48
Shared
(N – A)

79. Shared: Needed - AvailableF
Needed 8 56
Available 6 7 48
Shared
(N – A)

80. Shared: Needed - AvailableF
Needed 8 56
Available 6 7 48
Shared
(N – A)
8 shared 2
= 4 bonds

81. So I say why don’t you and I draw together
Each F can only have one bond, so S must be the central atom. Write S with the 6 F around it. Each F must be bonded to S at least once, so start drawing bonds in.

82. Take on the world, draw together forever
Uh oh! NAS says we can only have four bonds.

83. Heads we will, tails we draw again
We must be dealing with an expanded octet scenario. Since S is the only atom in the molecule with access to d orbitals, it must be the one expanding its octet.

85. Change S in the NAS chart to Need 10 valence electrons and recalculate.
BEFORE S F
Needed 8 56
Available 6 7 48
Shared
(N – A)
8 shared 2
= 4 bonds

86. Change Sulfur in the NAS chart to Need 10 valence electrons and recalculate.
AFTER S F
Needed 10 8 58
Available 6 7 48
Shared
(N – A)
10 shared 2
= 5 bonds

87. This tells us we need 5 bonds, which is still too few.
All the drawing people
This tells us we need 5 bonds, which is still too few.

88. Where do they all come from?
So we change S in the NAS chart to Need 12 valence electrons and recalculate.
BEFORE S F
Needed 10 8 58
Available 6 7 48
Shared
(N – A)
10 shared 2
= 5 bonds

89. All the drawing people
So we change S in the NAS chart to Need 12 valence electrons and recalculate.
AFTER S F
Needed 12 8 60
Available 6 7 48
Shared
(N – A)
12 shared 2
= 6 bonds

90. Where do they all belong?
This tells us we need 6 bonds, so we can finally connect each F to S.

91. Aaaaaahhhh, Look at all the drawing people
Then we need to put 36 electrons in lone pairs around the atoms. Fill each F's octet first, since each F MUST have 1 bond and 3 lone pairs.
Element 
# bonds 
# lone pairs  H  1  0  C  4  N  3  O  2 
All Halogens 
All Noble Gases  4

92. Aaaaaahhhh, Look at all the drawing people
Sulfur has more than a full octet. This is okay, because sulfur just expanded its octet by putting the extra electrons into its d orbitals.

93. Eleanor Rigby something something something drawing
Then, if there were extra electrons, they would go around S as lone pairs (although in this molecule there are not any because we have used all 48 available).

94. Don’t worry, it’s going to become useful now.
Example 5: SO22- Make an NAS chart. Need: 8 electrons each because neither S nor O are exceptions. Available: # valence BUT Don‘t forget that you have to include the charge of the ion in the # of available electrons!

95. Needed: 2 for H, 6 for B, 8 for everything else
charge
Needed 8
N/A 24
Available
Shared

96. Available: # valence electrons, including from the charge (negative charge adds electrons, positive charge removes electrons) S O
charge
Needed 8
N/A 24
Available 6 2 20
Shared

97. Shared: Needed - AvailableO
charge
Needed 8
N/A 24
Available 6 2 20
Shared
N – A 4

98. Shared: Divide by 2 to get the number of bonds
charge
Needed 8
N/A 24
Available 6 2 20
Shared
N – A
4 shared
= 2 bonds

99. When the night has drawn
S can expand its octet, which means it can have more bonds than O can. S must be the central atom. Write S with the two O around it. Each O must be bonded to S at least once, so draw one bond between each O and S. That takes care of both bonds for the molecule that the NAS chart requires.

100. And the land is dark
However, each O needs to have two bonds to be stable. This means we have to expand sulfur’s octet. Changing S to Need 10 e- will still only allot us three bonds, so we must change Sulfur to Need 12 e-.
BEFORE S O
charge
Needed 8
N/A 24
Available 6 2 20
Shared
N – A
4 shared
= 2 bonds

101. And the mooooon is the only light we draw
This will make the total needed be 28, making 8 shared e- and 4 bonds.
AFTER S O
charge
Needed 12 8
N/A 28
Available 6 2 20
Shared
N – A
8 shared
= 4 bonds

102. Oh I won’t be afraid
This takes care of all bonds by showing all 8 shared e-.

103. Nooooo I won’t be afraid
Since there are 20 e- available, the other 12 (unshared) e- need to be drawn in as lone pairs. Start by filling the octets of the O, since they are “normal” (not expanded octet) in this molecule.

104. Just as long
Doing so will give each oxygen 2 bonds and 2 lone pairs, which is its most stable configuration.
Element 
# bonds 
# lone pairs  H  1  0  C  4  N  3  O  2 
All Halogens 
All Noble Gases  4

105. As you draw
This takes care of 8 of the 12 remaining available electrons.

106. Draw by me
The final four must go in lone pairs somewhere. Since both O atoms have filled their octets and O cannot expand its octet (because it has no d orbital access), put the lone pairs on S.

107. So darlin’, darlin’, draw by me
This gives S 12 e- (8 in bonds, 4 in lone pairs), which agrees with our updated NAS chart.

108. Oooooh won’t you draw by me
Finally, since this is a polyatomic ion, draw square brackets around it and write a “-2” as the superscript charge.